1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine using an electrophotographic system.
2. Related Background Art
Conventionally, an image forming apparatus of a transfer system such as a copying machine, a printer, a facsimile machine using an electrophotographic system includes: an electrophotographic photosensitive member (photosensitive member) serving as an image bearing member, generally of a rotary drum type; an electrostatic charging device (charging step) for uniformly charging the photosensitive member to predetermined polarity and potential; an exposure device (exposure step) serving as information writing means which forms an electrostatic latent image on the charged photosensitive member; a developing device (developing step) which visualizes the electrostatic latent image formed on the photosensitive member as a developer image (toner image) with a toner serving as a developer; a transfer device (transfer step) which transfers the toner image from the surface of the photosensitive member to a transferring material such as paper; a cleaning device (cleaning step) which removes the developer somewhat remaining on the photosensitive member after the transfer step (residual toner, transfer residual toner) to clean the surface of the photosensitive member; and a fixing device (fixing step) which fixes the toner image on the transferring material. The photosensitive member is repeatedly subjected to an electrophotographic process (charging, exposure, development, transfer, and cleaning), serving for image formation.
Therefore, in the conventional image forming apparatus, a waste toner recovering container, receives the transfer residual toner removed from the surface of the photosensitive member by the cleaning device, is required, and maintenance for waste toner treatment is also required. In an image forming apparatus with a durable life thereof being set long, it is inevitable to either increase the number of times of maintenance or to increase the size of the waste toner recovering container. If the latter is selected, significant limitation is imposed in terms of reduction in size of the apparatus.
Thus, a cleaning-less image forming apparatus called “cleaning simultaneous with developing system” has been proposed, in which the cleaning device having the waste toner recovering container is removed and the transfer residual toner on the photosensitive member after the transfer step is removed and recovered by the developing device to be reused.
In the “cleaning simultaneous with developing system”, the photosensitive member is continuously charged and exposed to form an electrostatic latent image thereon while keeping the transfer residual toner after the transfer step on the photosensitive member. In the next development step, the transfer residual toner existing in a part on the photosensitive member which should not be developed (non-exposure part, non-image part) is recovered in the developing device with in accordance with “a fog removing bias (a fog removing potential difference Vback which is a potential difference between a DC voltage applied to the developing device and a surface potential of the-photosensitive member)”. Since the transfer residual toner recovered in the developing device by this system is reused in the development step in the following process, a waste toner is never generated. Thus, the maintenance conventionally required for treatment of the waste toner is never required, and the waste toner container can be removed. Therefore, the cleaning simultaneous with developing system is advantageous for reduction in size of the image forming apparatus.
On the other hand, in recent years, as the charging means, a roller charging system using an electroconductive roller as a contact charging member is particularly preferably used instead of a corona charger from the viewpoint of stability of charging. In the roller charging system, an electroconductive elastic roller (charging roller) is pressurized and brought into abutment against a member to be charged, and a voltage is applied thereto, thereby charging the member to be charged.
Concerning this charging system, as disclosed in Japanese Patent Application Laid-Open No. S63-149669, there is proposed an AC charging system in which a voltage is applied to the contact charging member. The voltage to be applied is obtained by superimposing an AC voltage component having a peak-to-peak voltage equal to or higher than 2×Vth (discharge initiating voltage) on a DC voltage equivalent to a desired surface potential Vd of the charged member. Such AC charging system is put to practical use. The AC charging system can realize more stable uniformization of charging due to a leveling effect of a potential by an AC voltage than the DC charging system using only a DC voltage.
In the cleaning-less image forming apparatus of the “cleaning simultaneous with developing system”, in the case in which the contact electrostatic charging device is used, when the transfer residual toner on the photosensitive member passes a contact portion between the photosensitive member and the contact charging member, a part of the transfer residual toner may attach on the surface of the contact charging member to contaminate the contact charging member. If such toner contamination of the contact charging member exceeds an allowable degree, charging failure may be caused.
The toner contamination worsens because a toner having a charged polarity reversed to a polarity opposite to a normal polarity (hereinafter referred to as “reversal toner”) or a toner, which has a low amount of charging and is hardly peeled from a photosensitive drum even if it is charged in the normal polarity (hereinafter referred to as “low charge toner”), exists in the transfer residual toner. This reversal toner or low charge toner electrostatically attaches on the contact charging member more easily than a toner of the normal polarity.
It is known that an existence ratio of the reversal toner or the low charge toner to the transfer residual toner increases due to an influence of a transfer bias voltage or peeling discharge in the transfer step compared with an existence ratio of the reversal toner or the low charge toner to the toner supplied for development from the developing device. This was a cause of worsening the toner contamination.
In addition, in order to remove and recover the transfer residual toner on the photosensitive member by “cleaning simultaneous with developing” with the developing device, a charged polarity of the transfer residual toner on the photosensitive member which is carried to a development portion is required to be a normal polarity, and an amount of charging thereof is required to be an amount of charging of the same level as a normal toner in the developing device. The reversal toner and the low charge toner in the transfer residual toner cannot be removed and recovered in the developing device from the photosensitive drum, causing image failure.
Thus, as disclosed in Japanese Patent Application Laid-Open No. H8-137368, there is proposed an image forming apparatus which is provided with a developer charging means (toner charging means) in the upstream of charging means, which charges a photosensitive member, in a rotational direction of the photosensitive member.
A DC voltage equal to or higher than a discharge initiating voltage in a normal polarity is applied to the toner charging means. A transfer residual toner passing the toner charging means is charged to the normal polarity by sufficient discharge. In the charging step performed by the contact charging member, the photosensitive member is charged together with the transfer residual toner. However, since the transfer residual toner is uniformly charged to the normal polarity by the toner charging means, the attachment of the transfer residual toner on the contact charging member is suppressed. In addition, since charge of the transfer residual toner is appropriately removed due to an AC voltage applied to the contact charging member, a toner having excessive charge and adhering to the photosensitive member with a reflection force is also removed. Removal and attachment of a toner is performed efficiently by “cleaning simultaneous with developing”.
However, although the transfer residual toner is given a sufficiently charge by the toner charging means if an amount of the transfer residual toner is little, a transfer residual toner image pattern may remain as it is to cause a ghost image thereof. In addition, under conditions that degrades transfer property of a toner image (e.g., high humidity environment, low resistance value of a transferring material, etc.), the amount of the transfer residual toner increases. When the transfer residual toner equal to or more than an allowable amount concentrates in a part of the toner charging means, development in which the toner charging means cannot fully control an amount of charging of the transfer residual toner in that part (development with transfer residual toner charging failure) may occur to cause a fogged image due to contamination of the charging member.
Thus, as disclosed in U.S. Pat. No. 6,421,512, the applicant of the present invention proposed an image forming apparatus provided with residual developer uniformizing means (residual toner uniformizing means) in the upstream of toner charging means and the downstream of a transfer portion. The residual toner uniformizing means disperses a pattern of a transfer residual toner image on a photosensitive member, which is carried to the toner charging means from the transfer portion, to unpattern the same. More specifically, the transfer residual toner image pattern is scraped or disturbed by rubbing the surface of the photosensitive member with a rubbing member, whereby the developer is dispersed over the surface of the photosensitive member. The dispersed transfer residual toner is sufficiently charged to the normal polarity by the toner charging means in the next step. Therefore, an effect of preventing the transfer residual toner from being attached on the contact charging member is improved significantly. By providing the residual toner uniformizing means, the generation of a ghost image or the generation of a fogged image due to contamination of the charging member as described above can be suppressed.
However, even in the case in which the toner charging means and the residual toner uniformizing means are provided, a toner (or an extraneous additive) may attach on the photosensitive member. Attachment of a toner is considered to occur as a result of a part of a toner (or an extraneous additive), which adheres to the surface of the photosensitive member due to discharge by the toner charging means, being exposed to discharge of the toner charging means and the contact charging member many times without being attached in the developing means and without being transferred by the transfer means. Such toner attachment corresponds to discharge unevenness in a longitudinal direction of the toner charging means and, in the case in which the toner charging means is constituted by an electroconductive brush or the like, occurs in a shape of sweeping seam of the brush along a rotational direction of the photosensitive member. The attached matter is easily scraped off by a conventional cleaning blade of a type that physically scrapes off the attached matter. However, the attached matter cannot be removed completely by the process of the cleaning-less type. Therefore, if the same photosensitive member is continuously used for a long period time, the attached matter accumulates to make the surface of the photosensitive member streak-like, which affects an image.
Thus, as disclosed in Japanese Patent Application Laid-Open No. 2001-215799, the applicant of the present invention made the toner charging means or the residual toner uniformizing means, or both of them to move in a longitudinal direction of the photosensitive member (a rotary shaft direction of the photosensitive member) such that discharge of the toner charging means does not concentrate on a specific region in the longitudinal direction of the photosensitive member while increasing an opportunity of the residual toner uniformizing means rubbing the photosensitive member, whereby the attached toner can be easily scraped off. In this way, the applicant of the present invention enabled occurrence and progress of attachment of the toner (or an extraneous additive) to be suppressed, preventing the toner from attaching to a specific portion on the photosensitive member. As a result, the toner, which is deposited at the same level, is dispersed substantially uniformly over the photosensitive member without any fluctuation, and a certain level of image quality can be maintained over a long period of time without involving any problem of image quality.
However, even if the toner charging means or the residual toner uniformizing means, or both of them are moved in the longitudinal direction of the photosensitive member, a portion with different state of attachment of the toner may occur periodically in the rotational direction of the photosensitive member according to a rotating period of the photosensitive member or a period for moving the toner charging means or the residual toner uniformizing means. There was a case where an influence of such a portion appeared in a halftone image or the like.